Digital telecommunication systems are currently evolving from the so-called first generation of narrow band networks, which are primarily directed to the handling of voice and data traffic, to a new generation of broad band networks which can carry a full range of multimedia services. Within a typical narrow band network, traffic and control information are carried in 64 kbit/s bearer channels using time division multiplexing (TDM). Routing to establish communications channels between end users is determined by the network nodes each of which is provided with a set of routing tables so as to set up an optimum route for each communication. The new broad band networks however are asynchronous in nature and carry traffic in the form of packets of cells each of which incorporates a header containing information whereby the packet is muted by the asynchronous swing fabric. Thus, if narrow band network traffic is to be carried over a broad band network, there is a problem of interfacing the narrow band circuit switched environment with the broad band packet environment. Further, there are differences in signalling protocols between the two types of network, and there is thus a need for a mechanism for carrying the narrow band signalling traffic over the broad band network such that the narrow band signalling remains fully functional.
A great deal of narrow band, typically voice, traffic is transported via the synchronous digital hierarchy (SDH) or the equivalent North American SONET protocol. Further legacy systems employ the plesiochronous digital hierarchy (PDH). In such systems, digitised traffic from a large number of users is packed into virtual containers which are multiplexed up into synchronous or plesiochronous frames prior to transmission. The transmission is time division multiplex (TDM) based. There is thus a problem in adapting this synchronous or plesiochronous bulk traffic for transport over a cell-based asynchronous network, and re-adapting the transported traffic from the cell-based ATM back to the original TDM-based synchronous or plesiochronous transport. This is becoming an increasing problem in current TDM transmission systems which are moving towards higher orders of multiplexing and correspondingly higher bit rates in an attempt to improve traffic handling capacity.
At present, this problem is addressed by the use of a residual time stamp technique (RTS) such as that described in U.S. Pat. No. 5,280,978. This technique provides a method and apparatus for recovering the timing signal of a constant bit rate input service signal at the destination node of a synchronous ATM telecommunication network. At the source node, a free-running P-bit counter counts cycles in a common network dock. At the end of every RTS period formed by N service dock cycles, the current count of the P-bit counter, defined as the RTS, is transmitted in the ATM adaptation layer. At the destination node, a pulse signal is derived in which the periods are determined by the number of network clock cycles represented by the received RTSs. This pulse signal is then multiplied in frequency by N to recover the source node service clock.
Whilst this technique provides a solution to the problem, it introduces significant complexity at the adaptation interface between the synchronous/plesiochronous and ATM networks.